This invention relates to liquid crystal devices. Such devices typically comprise a thin layer of a liquid crystal material contained between glass slides. Electrode structures on the slides allow an electric field to be applied across the layer causing a re-ordering of the liquid crystal molecules to an ON-state. On removing the electric field the molecules relax back to their OFF state.
There are three known types of liquid crystal material, nematic, cholesteric, and smectic each having a different molecular ordering.
The present invention utilises a long-pitch cholesteric mixture in which the molecules naturally adopt a helical structure with the helix axes normal to the plane of the layer. The long-pitch cholesteric mixture is formed by mixing typically a few percent of a cholesteric liquid crystal material with a nematic liquid crystal. The pitch of the cholesteric mixture can be controlled by adjusting the concentration of the cholesteric liquid crystal material.
One known device is the twisted nematic device which uses a thin layer of a nematic material between glass slides. The slides are unidirectionally rubbed and assembled with the rubbing directions orthogonal. The rubbing gives a surface alignment to the liquid crystal molecules resulting in a progressive 90.degree. twist across the layer. When placed between polarisers, with their optical axis perpendicular or parallel to a rubbing direction the device rotates the plane of polarised light in its OFF state and transmits without rotation in the ON state. Small amounts of cholesteric have been added to the nematic material to ensure the 90.degree. twist is of the same sense across the whole area of the device as explained in U.K. Pat. Nos. 1,472,247 and 1,478,592.
A known type of device employing short-pitch cholesteric material is the phase change device. In its OFF, zero applied voltage, state the material is scattering to light. The cholesteric pitch is very small in comparison to the layer thickness. When an above threshold voltage is applied across the layer its molecules rotate from their helical structure to lie parallel to the applied field. This is similar to the ON state of a positive nematic and is light transparent. On removing the voltage the material relaxes back to its light scattering OFF state. No cell wall surface alignment is necessary with this type of device. An advantage of this device is the fast turn OFF time compared with the twisted nematic device and wide angle of view.
Another type of cell utilises a cholesteric material whose pitch equals the layer thickness. With homogeneous boundary conditions a plan state is induced with a 2.pi. rotation of the director across the layer. Inclusion of a pleochroic dye produces absorption of light in the OFF state. Application of a voltage re-orientates the liquid crystal and dye molecules giving higher transmission in the ON state. As the voltage is increased to just above threshold a scattering texture is formed, which clears after a further, substantial, increase of voltage to give the ON state. As the voltage is reduced there is a rapid reduction in transmission at the threshold voltage. The device therefore always shows significant hysteresis which is present irrespective of the rate of increase of voltage. This hysteresis limits the multiplexibility of this device. This device is described in I.E.E.E. Trans. on Electron Devices, Vol. ED 28 No. 6, June 1981, pp. 719-823.
It has now been found that certain cholesteric liquid crystal cells incorporating a pleochroic dye and having about a 3.pi./2 twist exhibit a sharp transmission-voltage characteristic for rapidly increasing voltages, without hysteresis. A slow increase in voltage may still produce some hysteresis.
According to this invention a liquid crystal device comprises a layer of long pitch cholesteric liquid crystal material of positive dielectric anisotropy and incorporating an amount of a pleochroic dye contained between two cell walls bearing electrode structures and being surface treated to align liquid crystal molecules in a tilted homogeneous structure characterised in that the surface alignment and natural pitch p of the cholesteric material are matched to provide a progressive molecular twist of greater than .pi. and less than 2.pi. radians across the layer with a uniform tilt direction, the ratio of layer thickness d divided by pitch p lying between 0.5 and 1.0 with a value of d less than 20 .mu.m whereby the device may be switched directly between a light transmissive ON state and a non transmissive OFF state with a sharp transmission/voltage chracteristics without substantial hysteresis.
The amount of twist is preferrably around 3.pi./2. The surface alignment treatment results in a molecular tilt of between 0.degree. and 30.degree. or more but less than 90.degree. (hemeotropic alignment). A tilt of less than say 5.degree. e.g. 2.degree. may be termed low tilt and between say 5.degree. and 70.degree. termed a high tilt. Both low and high tilt may be used singly or in combination.
The device may operate in a transmissive mode or with a reflector in a reflective mode both with or without neutral or coloured polarisers.
Two or more devices may be arranged in series. This arrangement may use different colour dyes in each device so that a total of three colours may be observed. Alternatively the dyes may be similar so that a single colour of enhanced contrast between OFF and ON states is obtained. Furthermore each cell may have a different absorption so that eight values of absorption or grey scale are obtained.
Usually the pitch of a cholesteric liquid crystal increases with increasing temperature. The device may be enhanced by either
(i) choosing a liquid crystal material, or mixture of materials, with small or zero temperature dependence of pitch to maintain the optimum d/p over a wide temperature range; or PA1 (ii) choosing a liquid crystal whose pitch decreases with increasing temperature to partially or wholly compensate for variations in threshold voltage with temperature.
Additionally, or instead, the liquid crystal temperature may be sensed and the addressing voltage levels altered accordingly. One method of sensing is described in U.K. Pat. No. 2,012,093 B.